QUANTUM-CHEMICAL PREDICTIONS OF THE ELECTRON-AFFINITIES OF CARBON-HYDROGEN CLUSTERS C2NH-CENTER-DOT, THE CH BINDING-ENERGIES AND THE GAS-PHASE ACIDITIES OF POLYACETYLENES C2NH2 FOR N=1-3

Accurate data for the adiabatic electron affinities of the radicals C2nH., the CH bond dissociation energies and the gas phase acidities of the polyacetylenes C(2n)H-2 for n = 1-3 have been obtained using the complete active space SCF approach to optimize the geometries and coupled cluster, and for some cases multi-reference configuration interaction and averaged coupled pair functional methods to refine the energies in connection with the polarized basis sets due to Sadlej and the generally contracted atomic natural orbitals basis sets, respectively. Harmonic frequencies have been computed for all species using a density functional theory/Hartree-Fock hybrid approach employing the Becke3LYP functional and the 6-311G** basis set. Our final theoretical predictions for the electron affinities are 2.96 +/- 0.04, 3.46 +/- 0.07 and 3.69 +/- 0.07 eV for C2H., C4H. and C6H., respectively. For the CH binding energies and gas phase acidities we predict D-0 = 131.7 +/- 1.2, 130.2 +/- 2.0, and 127.5 +/- 2.0 kcal mol(-1) and Delta G(acid,298) = 369.5 +/- 2.1, 356.3 +/- 3.6 and 348.7 +/- 3.6 kcal mol(-1) for C2H2, C4H2 and C6H2, respectively. Where available, these data compare very well with experimental results. In addition, the question of the ground state of the C4H. radical has been settled. It is of 2 Pi symmetry, with the (2) Sigma(+) state lying only some 3 kcal mol(-1) higher in energy. Hence there is no 'alternation rule' for the ground state of the species CnH. as all of them except C2H. assume a (2) Pi ground state.